Hearing protector attenuation: A perspective view

The design, performance and evaluation of hearing protectors are matters of substantial current interest. The specific scientific questions which command attention include the accuracy of the shift in free field hearing threshold as a measure of protection, the relationship between physical measurements of sound attenuation with real heads and psycho-physical methods of measurement, the relationship between physical measurements with real and artificial heads, the acoustical behaviour of hearing protectors and the factors which limit their performance, and the relationship between hearing protector attenuation and speech intelligibility. These questions have received considerable attention during the past twenty years but are not yet fully answered.     

Measuring hearing in the harbor seal (Phoca vitulina): comparison of behavioral and auditory brainstem response techniques

Auditory brainstem response (ABR) and standard behavioral methods were compared by measuring in-air audiograms for an adult female harbor seal (Phoca vitulina). Behavioral audiograms were obtained using two techniques: the method of constant stimuli and the staircase method. Sensitivity was tested from 0.250 to 30 kHz. The seal showed good sensitivity from 6 to 12 kHz [best sensitivity 8.1 dB (re 20 microPa2 x s) RMS at 8 kHz]. The staircase method yielded thresholds that were lower by 10 dB on average than the method of constant stimuli. ABRs were recorded at 2, 4, 8, 16, and 22 kHz and showed a similar best range (8-16 kHz). ABR thresholds averaged 5.7 dB higher than behavioral thresholds at 2, 4, and 8 kHz. ABRs were at least 7 dB lower at 16 kHz, and approximately 3 dB higher at 22 kHz. The better sensitivity of ABRs at higher frequencies could have reflected differences in the seal's behavior during ABR testing and/or bandwidth characteristics of test stimuli. These results agree with comparisons of ABR and behavioral methods performed in other recent studies and indicate that ABR methods represent a good alternative for estimating hearing range and sensitivity in pinnipeds, particularly when time is a critical factor and animals are untrained.     

Methods of measuring the attenuation of hearing protection devices

The published literature describing three real-ear-attenuation-at-threshold (REAT), nine above-threshold, and four objective methods of measuring hearing protector attenuation is reviewed and analyzed with regard to the accuracy, practicality, and applicability of the various techniques. The analysis indicates that the REAT method is one of the most accurate available techniques since it assesses all of the sound paths to the occluded ear and, depending upon the experimenter's intention, can reflect actual in-use attenuation as well. An artifact in the REAT paradigm is that masking in the occluded ear due to physiological noise can spuriously increase low-frequency (less than or equal to 500 Hz) attenuation, although the error never exceeds approximately 5 dB, regardless of the device, except below 125 Hz. Since the preponderance of available data indicates that attenuation is independent of sound level for intentionally linear protectors, the use of above-threshold procedures to evaluate attenuation is not a necessity. An exception exists in the case of impulsive noises, for which the existing data are not unequivocal with regard to hearing protector response characteristics. Two of the objective methods (acoustical test fixture and microphone in real ear) are considerable time savers. All objective procedures are lacking in their ability to accurately determine the importance of the flanking bone-conduction paths, although some authors have incorporated this feature as a post-measurement correction. The microphone in real-ear approach is suggested to be one of the most promising for future standardization efforts and research purposes, and the acoustical test fixture technique is recommended (with certain reservations) for quality control and buyer acceptance testing.     

A signal generator for testing of hearing protectors

The measurement of the acoustical attenuation of hearing protectors requires a signal generator whose characteristics are specified in the ANSI S3.19-1974 Standard. This paper describes such a generator which was constructed by modifying a regular screening audiometer, including a pink noise generator followed by nine one-third octave band filters and an additional 2·5 dB attenuator. Detailed information on the construction and performance of the generator is provided in the paper. The generator, which is relatively easy to build, has proved to be reliable for uses in the laboratory, as well as in the field.     

Selection of hearing protectors

A method for selection of hearing protectors based on attenuation and comfort is presented. For the attenuation, the data provided by the manufacturer are used after being derated. A protector is selected when its attenuation is higher than the measured (or calculated) noise exposure minus the criterion level set by regulations or legislation. When large numbers of workers are involved, group noise exposure and associated standard deviation are to be used.Comfort is assessed by using a specially developed questionnaire. Protectors are distributed to ten workers who have to wear them during an entire shift. The questionnaire is completed prior to and after wearing the protectors.     

Real-ear attenuation of earmuffs in normal-hearing and hearing-impaired individuals

Many of the 9 million workers exposed to average noise levels of 85 dB (A) and above are required to wear hearing protection devices, and many of these workers have already developed noise-induced hearing impairments. There is some evidence in the literature that hearing-impaired users may not receive as much attenuation from hearing protectors as normal-hearing users. This study assessed real-ear attenuation at threshold for ten normal-hearing and ten hearing-impaired subjects using a set of David Clark 10A earmuffs. Testing procedures followed the specifications of ANSI S12.6-1984. The results showed that the hearing-impaired subjects received slightly more attenuation than the normal-hearing subjects at all frequencies, but these differences were not statistically significant. These results provide additional support to the finding that hearing protection devices are capable of providing as much attenuation to hearing-impaired users as they do to normal-hearing individuals.     

Development of a new standard laboratory protocol for estimation of the field attenuation of hearing protection devices: sample size necessary to provide acceptable reproducibility

The mandate of ASA Working Group S12/WG11 has been to develop "laboratory and/or field procedure(s) that yield useful estimates of field performance" of hearing protection devices (HPDs). A real-ear attenuation at threshold procedure was selected, devised, tested for one earmuff and three earplugs via an interlaboratory study involving five laboratories and 147 subjects, and incorporated into a new standard that was approved in 1997 [Royster et al., "Development of a new standard laboratory protocol for estimating the field attenuation of hearing protection devices. Part I. Research of Working Group 11, Accredited Standards Committee S 12, Noise," J. Acoust. Soc. Am. 99, 1506-1526; ANSI, S12.6-1997, "American National Standard method for measuring real-ear attenuation of hearing protectors" (American National Standards Institute, New York, 1997)]. The subject-fit methodology of ANSI S12.6-1997 relies upon listeners who are audiometrically proficient, but inexperienced in the use of HPDs. Whenever a new method is adopted, it is important to know the effects of variability on the power of the measurements. In evaluation of protector noise reduction determined by experimenter-fit, informed-user-fit, and subject-fit methods, interlaboratory reproducibility was found to be best for the subject-fit method. Formulas were derived for determining the minimum detectable difference between attenuation measurements and for determining the number of subjects necessary to achieve a selected level of precision. For a precision of 6 dB, the study found that the minimum number of subjects was 4 for the Bilsom UF-1 earmuff, 10 for the E.A.R Classic earplug, 31 for the Willson EP100 earplug, and 22 for the PlasMed V-51R earplug.     

Simplified hearing protector ratings—an international comparison

A computer was programmed to model the distributions of dB(A) levels reaching the ears of an imaginary workforce wearing hearing protectors selected on the basis of either octave band attenuation values or various simplified ratings in use in Australia, Germany, Poland, Spain or the U.S.A. Both multi-valued and single-valued versions of dB(A) reduction and sound level conversion ratings were considered. Ratings were compared in terms of precision and protection rate and the comparisons were replicated for different samples of noise spectra (N = 400) and hearing protectors (N = 70) to establish the generality of the conclusions. Different countries adopt different approaches to the measurement of octave band attenuation values and the consequences of these differences were investigated. All rating systems have built-in correction factors to account for hearing protector performance variability and the merits of these were determined in the light of their ultimate effects on the distribution of dB(A) levels reaching wearers' ears. It was concluded that the optimum rating is one that enables the dB(A) level reaching wearers to be estimated by subtracting a single rating value from the dB(C) level of the noise environment, the rating value to be determined for a pink noise spectrum from mean minus one standard deviation octave band attenuation values with further protection rate adjustments being achieved by the use of a constant correction factor.     

Is it necessary to measure hearing protector attenuation at 3.15 and 6.3 kHz?

The real-ear attenuation data for 81 different hearing protectors were analyzed with respect to the errors that would arise if, instead of averaging the 1/3-octave-band results at 3.15 and 4 kHz and 6.3 and 8 kHz, respectively, the octave-band attenuation at 4 and 8 kHz was estimated from only the 1/3-octave-band data at those two frequencies. Errors as large as 3-4 dB were found to occur in rare instances, but more typically were in the range of 0.5-1.5 dB. However, in terms of computation of an overall noise reduction rating such as the NRR, the effect of excluding the 3.15- and 6.3-kHz data led to errors that averaged only 0.1 dB and never exceeded 0.3 dB, except in one instance, where the error was 0.6 dB. It was concluded that there is little value in measuring real-ear attenuation in a diffuse sound field at the frequencies of 3.15 and 6.3 kHz for applications in which hearing protector attenuation data are normally utilized.     

MIRE-IL methodology applied to measuring the noise attenuation of earmuff hearing protectors

This article describes an objective methodology for measuring the noise attenuation of earmuff hearing protectors using as a reference the method known as microphone-in-real-ear (MIRE). The methodology implements the insertion loss (IL) paradigm, in which IL is measured using miniature microphones, specially designed to comply with ANSI and ISO standards for the MIRE technique. The results for four different hearing protectors are compared with the subjective method known as real-ear-attenuation-at-threshold (REAT). Correction factors are included in the methodology to account for external effects such as physiological noise and bone conduction. The objective method predicted well the real protection of the earmuffs and the proposed methodology showed lower standard deviation values than the REAT method.     

Attenuation of hearing protectors for pure-tones and 13-octave bands of noise

Eleven hearing protectors were tested according to the Argentinian Standard (IRAM 4060) with pure tones and random noise filtered in 1/3-octave bands as the test stimuli. Although the differences between the standard deviations are not statistically significant, there are significant differences between mean values for the two types of test signal. This indicates that a re-examination of the standard method is required.     

Use of the sound levels of noise for assessing the adequacy of hearing protectors

The dB(A) sound level of a noise is accepted as a measure of the damage risk to unprotected ears but often it is not a reliable guide to the risk to ears fitted with hearing protectors. For any dB(A) level inside a protector, normally there will be substantially higher sound levels outside that protector. This paper shows how, from sequential frequency attenuation bands of the protector, and sound level weightings, external sound levels can be calculated, below which the noise inside the protector does not exceed a chosen dB(A) level. Further valuable information may be obtained by mapping external dB(A) and dB(C) levels to cover all possible noise spectra that give the chosen dB(A) level inside the protector. Thus, from a pair of measured sound levels, use of the method indicates whether the protector is sufficient or not, or whether more detailed measurment of the noise is required. This knowledge enhances the scope of the sound level meter and reduces the need for frequency analysis of industrial noise. Its application should be a helpful addition to the data provided by suppliers of hearing protectors.     

A field study to assess the effects of wearing hearing protectors on the perception of warning sounds in an industrial environment

A field study has been performed which assessed the effectiveness of two warning sounds under realistic factory conditions. The thirty subjects were press operators who were involved in their regular work activities during the course of the experiment. The noise levels at their work stations were in the range 85 to 95 dB(A). The warning sound, plus four other irrelevant sounds, each at one of five different intensities, were presented via a loudspeaker in random order and at random time intervals (mean inter-stimulus interval, 30 s; range, 10 to 50 s). The influence of the factors hearing level and age of the subjects and use of hearing protection were investigated.The results showed significant differences between the intentional and incidental warning sounds investigated. Whilst the perception of the intentional horn warning sound was unaffected by the hearing sensitivity of the subjects and the wearing of hearing protectors, these two factors had a significant effect on the perception, as a warning, of the clinking sound of metal components. Subjects classified as having a substantial hearing loss perceived, on average, 18 percent fewer clinking sounds than those subjects with normal hearing or a mild hearing loss and, when wearing hearing protectors, the subjects overall perceived, on average, 9 percent fewer clinks. It is of note that the results of the field study differ from those of earlier laboratory-based experiments in finding that the distinctive intentional warning sound was not entirely reliable as a warning, even at levels substantially above its masked threshold, and in observing more than double the adverse effect of wearing hearing protection.     

Design of a pulse generator and shock tube for measuring hearing protector attenuation of high-amplitude impulsive noise

The effectiveness of hearing protectors against high amplitude impulse noise levels remains the subject of research with objective testing techniques using acoustic test fixtures offering the only realistic method of providing rapid performance data for protector design and qualification. The work presented in this paper examines a prototype test method based on a shock tube and acoustic test fixture for the evaluation of protectors against high-level impulsive noise where established real ear attenuation at threshold methods would be impractical to apply. The results show that the system is capable of producing controlled repeatable high amplitude pressure pulses of variable duration for testing hearing protection devices in a grazing wave type test. A series of pilot tests illustrate how the system can have a sufficient self-insertion loss to reject flanking noise and allow the measurement of protector attenuations of up to 45 dB with little corruption from flanking noise.     

Sound attenuation of TDH-39 earphones in a diffuse field of narrow-band noise

The sound attenuation characteristics of the audiometric earphone Telephonics TDH-39 with cushion MX-41/AR have been tested, using the hearing threshold method according to the International Standard ISO 4869-1981 ("Acoustics--Measurement of sound attenuation of hearing protectors--Subjective method"). This method specifies 1/3-oct bands of noise as test signals and a diffuse sound field. This is in contrast to the use of pure tones in a frontally incident free sound field, by means of which most earlier attenuation data have been obtained. The mean sound attenuation values obtained from a group of 20 normal-hearing subjects were up to 3-5 dB lower than those previously reported for pure tones in a free field.     

Effects of hearing protector devices on speech intelligibility

The purpose of this study was to determine the influence of hearing protection devices (HPDs) on the understanding of speech in young adults with normal hearing, both in a silent situation and in the presence of ambient noise. The experimental research was carried out with the following variables: five different conditions of HPD use (without protectors, with two earplugs and with two earmuffs); a type of noise (pink noise); 4 test levels (60, 70, 80 and 90 dB[A]); 6 signal/noise ratios (without noise, +5, +10, zero, −5 and −10 dB); 5 repetitions for each case, totalling 600 tests with 10 monosyllables in each one. The variable measure was the percentage of correctly heard words (monosyllabic) in the test. The results revealed that, at the lowest levels (60 and 70 dB), the protectors reduced the intelligibility of speech (compared to the tests without protectors) while, in the presence of ambient noise levels of 80 and 90 dB and unfavourable signal/noise ratios (0, −5 and −10 dB), the HPDs improved the intelligibility. A comparison of the effectiveness of earplugs versus earmuffs showed that the former offer greater efficiency in respect to the recognition of speech, providing a 30% improvement over situations in which no protection is used. As might be expected, this study confirmed that the protectors' influence on speech intelligibility is related directly to the spectral curve of the protector's attenuation.     

Design of a reverberant chamber for noise exposure experiments with small animals

Previous work on the acoustic design of small reverberant chambers for studies on laboratory animals has paid, in general, more attention to the frequency response at certain points in their interior. These designs aimed to provide a frequency response as flat as possible at the receivers, thus avoiding unpleasant spectral coloration effects. However, an equally important, and frequently neglected, aspect is to set an acoustic field as spatially uniform as possible inside the zone where the animals are to be placed during the exposure to noise. Here, an optimization procedure is described to calculate the proportions of the chamber dimensions that confers the highest sound level with the minimum mean squared deviation averaged in a given area inside the chamber. In addition, new stimuli have been designed with a high-pass filtering and linear with frequency gain. These stimuli were intended to adapt the characteristics of the exposing noise to the rodent hearing spectrum, displaced toward higher frequencies than the hearing frequency band of humans.     

Correlation among thresholds of evoked otoacoustic emissions,behavioral responses and auditory brainstem responses

In 35 adult human subjects(58 ears)thresholds of rarefactlon clickevoked otoacoustic emission(EOAE),behavioral response(BR)and auditorybrainstem response(ABR)were measured and compared,and correlationcoefficients(r)among them calculated.The results revealed that 86％ of themeasured thresholds were in the range from 10 to 45 dB(nHL)for EOAE,88％ from 10 to 25 dB for BR and 88％ from 10 to 35 for ABR.The correlationcoefficients for the whole threshold sample were 0.415(p<0.002)for EOAE vs.BR,0.501(p<0.001)for EOAE vs.ABR and 0.702(p<0.001)for ABR vs.BR,all indicating highly significant correlation.However,for those ears whoseBR,ABR or EOAE thresholds were elevated,equaling to or exceeding 25 dB,there is no significant correlation between thresholds of EOAE and BR and ofEOAE and ABR(r range:0.176-0.310,p>0.05).In contrast,significantcorrelation between BR and ABR thresholds in the same conditions still re-mained(r:0.533-0.720,p<0.05).The experimental results indicate that thecorrelation between EOAE and hea     

Speech production in noise with and without hearing protection

People working in noisy environments often complain of difficulty communicating when they wear hearing protection. It was hypothesized that part of the workers' communication difficulties stem from changes in speech production that occur when hearing protectors are worn. To address this possibility, overall and one-third-octave-band SPL measurements were obtained for 16 men and 16 women as they produced connected speech while wearing foam, flange, or no earplugs (open ears) in quiet and in pink noise at 60, 70, 80, 90, and 100 dB SPL. The attenuation and the occlusion effect produced by the earplugs were measured. The Speech Intelligibility Index (SII) was also calculated for each condition. The talkers produced lower overall speech levels, speech-to-noise ratios, and SII values, and less high-frequency speech energy, when they wore earplugs compared with the open-ear condition. Small differences in the speech measures between the talkers wearing foam and flange earplugs were observed. Overall, the results of the study indicate that talkers wearing earplugs (and consequently their listeners) are at a disadvantage when communicating in noise.     

Auditory brainstem response recovery in the dolphin as revealed by double sound pulses of different frequencies.

Recovery of auditory brainstem responses (ABR) in a bottlenose dolphin was studied in conditions of double-pip stimulation when two stimuli in a pair differed in frequency and intensity. When the conditioning and test stimuli were of equal frequencies, the test response was markedly suppressed at short interstimulus intervals; complete recovery appeared at intervals from about 2 ms (when two stimuli were of equal intensity) to 10-20 ms (when the conditioning stimulus exceeded the test by up to 40 dB). When the two stimuli were of different frequencies, the suppression diminished and was almost absent at a half-octave difference even if the conditioning stimulus exceeded the test one by 40 dB. Frequency-dependence curves (ABR amplitude dependence on frequency difference between the two stimuli) had equivalent rectangular bandwidth from +/-0.2 oct at test stimuli of 20 dB above threshold to +/-0.5 oct at test stimuli of 50 dB above threshold.